Sound recording and reproducing



April 7, 1931. J. w. HORTON 1,799,795

SOUND RECORDING AND REPRODUCING Filed y 5. 1928 2 Sheets-Sheet 1 Ha. I.

April 7, 1931.

J. w. HORTON 1,799,795

SOUND RECORDING AND REPRODUCING Filed May 5, 1928 2 Sheets-Sheet 2 llllll l/UA/Tfi/ER FREQUENCY H6. 5.

"IX'PHASE CHARACTER/5776 OF /"/L 715/? I l i I I l I l I FREQUENCY wmv 70/? JOSEPH W /70R ro/v By Patented Apr. 7, 1931 UNITED STATES PATENTOFFICE JOSEPH W. HORTON, OF MILLBURN, NEW/V JERSEY, ASSIGNOR TO BELLTELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATIONOF NEW YORK Application fil d May 5,

19, 1926, there is disclosed a recording and reproducing system whereinspeech to be recorded is used to modulate a carrier. Only the lower sideband is recorded so that in the resulting record the frequency order isinverted and the whole frequency band may be displaced by an amountdepending upon the frequency of the carrier employed. Such a record 18of course unlntelllgible when reproduced in the ordinary way but whenthe same carrier frequency is supplied in the reproducing circuit theoriginal speech frequencies may be reproduced in proper order asdescribed. The system proposed by Mills is intended to be used as amethod of making secret records.

The object of the present invention is to adapt such a system to theproduction of high quality records.

In accordance with the general features of the invention, the band offrequencies representing the sound to be recorded is dis placedas wellas inverted on the frequency scale in such way that the percentageinterval between the upper and lower limits is reduced so that therecording and reproducing mechanisms may be made more uniformlyresponsive to the frequencies involved. Elcctromagnetic means associatedwith the recording and reproducing turntables generate carriers whichvary in frequency with the speed of the turntable so that any speedvariation will shift the phase of the wave components in proportion totheir frequency as in the ordinary phonograph. For special purposes analternative system employing a balanced pick-up device and doublemodulation permits the recording of all frequencies down to zero.

The invention may be more fully understood from the following detaileddescription when read in connection with the accompanying drawings.Figs. 1 and 2 of the drawings show recording and reproducing systemssuitable for making and reproduc- SOUND RECORDING AND REPRODUCING SerialNo. 275,409.

ing high quality records of speech or music in accordance with onefeature of this invention; Fig. 3 shows a special system which may beused for making records of sound frequencies extending down to zero;Fig. 4 shows a relatively simple reproducing system for such specialrecords; and Fig. 5 illustrates the characteristics of the specialfilter required for the system shown in Fig. 3.

Referring now to Fig. 1, electron discharge devices 1, 1 are arranged toact as a balanced modulator of a well known type, such as that disclosedin Fig. 8 of Patent 1,343,306 to J. R. Carson, granted June 15, 1920. Acoil 2 is wound on a U-shaped laminated core 3 which is adjustablyattached to the frame 4 of the recording phonograph by a clamping device5. The turntable 6 is composed of magnetic material slotted at itsperiphery to form alternate slots and pole pieces of equal width, asindicated. The pole faces of the U-shaped core are preferably equal inwidth to those on the turntable and sufficiently separated from them togive the necessary operating clearance. The harmonic generator 7 may beof the type disclosed in Patent 1,446,752, granted to Kendall, February27, 1923, and the filter 8 may be merely a tuned circuit arranged topass to the input transformer 9 only a very narrow band including theharmonic frequency desired for the carrier in the recording system. Thenumber of slots in the turntable may vary greatly but is preferably aslarge as is consistent with good mechanical practice in order that theproper carrier frequency may be more easily obtained from the harmonicgenerator.

The necessary exciting current for the coil 2 may be supplied by abattery or other source associated with the harmonic generator 7, or asan alternate construction, the U shaped core may be a permanent magnetand then no exciting current would be necessary. In either case therotation of the turntable will cause alternating currents to begenerated in the coil. The frequency of these currents will depend uponthe number of slots employed and will, of course, vary directly withchanges in the speed of rotation. From this primary frequency thedesired carrier frequency is obtained by means of the harmonic generatorand is passed to the modulating system by the filter circuit 8.

In the ordinary phonograph the frequency generated by any givencomponent of the recorded wave is a function of the speed of rotationand any change in speed raises or lowers the frequency of each componentby the same percentage of its original value so that their relativephase positions are not disturbed and the quality is not seriouslyimpaired for small fluctuations. In the system of the present invention,however, the sound actually reproduced is of a pitch correspond ing tothe numerical difference between the carrier frequency and the componentbeing reproduced at the time.

If the turntable speed varies somewha from its proper speed, it will beevident that the carrier frequency must vary in the same manner or thechange in pitch of the sound reproduced will be different for eachcomponent and the reproduction will be discordant.

The carrier generating device shown provides a simple method ofobtaining a carrier of a frequency which is varied automatically tocompensate for variations in turntable speed.

The function of the adjustable clamping device 5 may be brieflydescribed as providing an easy means of bringingthe carri r into phasewith the components of carrier frequency in the inverted wave form. Thisadjustment becomes important when recording very low frequencies andwill be further described in connection with Fig. 4. In other respectsthe systems of Figs. 1 and 2 are well known in the art so that nofurther description is required.

Heretofore it has been customary in systems for inverting speechfrequencies to employ a carrier having a frequency of the order of thehighest frequency to be inverted so that the lower side band occupiedsubstantially the same position in the frequency scale as the uninvertedfrequencies. According to one feature of this invention the frequencyratio or percentage interval of the band to be recorded is reduced byemploying a carrier of a frequency considerably higher than the upperlimit of the band.

Assume for purposes of illustration that records are to be made ofspeech and music frequencies from (30 to (5000 cycles. With ordinarymethods such records will have a frequency ratio of 100 to 1. It is verydifficult to design recording and reproducing mechanisms which willoperate with even an approximately linear characteristic over so wide arange. When the band is displaced, however, by employing a carrier of7060 cycles for instance. the resulting lower side band will consist offrequencies between 1060 and 7000 cycles which reduces this ratio toless than 7 and makes it possible to record and reproduce at practicallyconstant efficiency.

.Vith circuits arranged in the manner shown in Figs. 1 and 2, it is Wellknown in the art that the currents of speech frequencies setup intransformer 10 by the action of sounds upon microphone 11 are balancedout in the plate circuit of the modulator and hence do not appear uponthe record. Since the upper side band consists of frequencies between7120 and 13060 cycles, it may be eliminated by designing the recorder 12to be nonresponsive to frequencies much in excess of 7000 cycles. Inthis way, the record made will include only the lower side band offrequencies between 1060 and 7000 and possibly a slight amount of thecarrier which can be so nearly eliminated by the recorder as to beinaudible.

The reproducing system of Fig. 2 is practically identical with therecording system just described. T he recorded inverted speechfrequencies are picked up by reproducer 13 and translated into electriccurrents which traverse the primary of transformer 142. A

7000 cycle carrier obtained as in Fig. 1 is also supplied to themodulator 15 and causes a reinversion of the frequency order to producea lower side band of 606000 cycles in the output circuit. The windingsof receiver 16 may be inserted directly in the output circuit asindicated to balance out in the magnetic structure of the receiver themagnetomotive forces due to space current. In this way the signaldistortion is further reduced by the elimination of the usual outputtransformer.

The special system of Fig. 3 is similar in a general way to that of Fig.1, but different in certain important respects on account of the specialrequirements of a system which is to be responsive to frequenciesapproaching zero. The double button pick-up device is energized bybattery 18 and resistance coupled to the modulator 20 by resistances 21and 22. Vith this construction all frequencies extending down to zeroare effective to control the modulator.

This is readily seen from a consideration of the extreme case of zerofrequency or a constant acoustic pressure in one direction. In such acase the transmitter is held continuusly in one position so that adirect currenttraverses the closed loop circuit which includes one ofthe resistances 21 or 22 and one side of the transmitter. During thistime the other loop circuit including the other of the resistances iscarrying a greatly reduced current so that the grid potentials of thetubes comprising the modulator 20 are no longer equal. Under thiscondition the carrier frequency is not entirely balanced out in theplate circuit and is therefore transmitted as a representation of zerofrequency.

The carrier frequencies used in this system are generated in the mannerpreviously described for the system of Figs. 1 and 2. In this case,however, the filter 23 is adapted to pass a higher frequency'carriersuch as 16000 cycles and this frequency is applied tothe modulatorthrough transformer 24. Since it is proposed in this case to recordall'frequencies between .0 and 6000 cycles, the lower side band willconsist of frequencies between 10000 and 16000 cycles and thecorresponding upper side band of'frequencies between 16000 and 22000cycles. In the secondary circuit of output transformer 25 a band passfilter 26 is adapted to cut 03 all frequencies below say 8000 cycles soas to eliminatethe undesired products of modulation, such as theoriginal sound frequencies. 0

Both side bands are therefore impressed upon the input circuit of asecond modulator 27 which may be of any well known type, throughtransformer 28. The filter 20 is arranged topass an 8000 cycle carrierto the modulator 27 so that the lower side band in the output circuitwill consist of frequencies from 2000 to 14000. The upper side band willrange from 18000 to 30000 which is of course beyond the limit ofresponse of the system.

One further feature of this system should be particularly described.Since there is no separation between the upper and lower side bandsinthe output of'modulator 20, it is physically impossible to record thedesired frequencies delivered by modulator 27, namely 2000 to 8000,without also recording some part of the band from 8000 to 14000 whichrepresent the upper side band of the first carrier. This condition ismet according to this invention'by inserting'in the recorder circuit aspecial filter 30, which is designed to have a characteristic asrepresented by the diagram of Fig. 5. From the full line representin gthe amplitude of the currents of various frequencies passed by thisfilter, it is apparent that the major portion of the upper side band hasbeen eliminated. In order to prevent the unsuppressed portion of theside band from passing to the recorder 31 and thereby introducingdistortion into the record, the phase characteristic of the filter is astraight line through the intersection of the carrier frequency and thefilter characteristic. With a filter of this type, the unsuppressedsignal components above carrier frequency will be in phase with thecorresponding components below that frequency, so that the resultingrecord will be substantially the same as if the upper side band had beenentirely eliminated.

By way of a brief review of what has just been described, it is pointedout that the original band of sound frequencies GXtQIlCl ing from 6000down to zero was first displaced upward in the frequency scale by the16000 cycle carrier where it appeared as two side bands ranging from10000 to 22000 cycles. This displacement made it possible to effectivelyeliminate the original band by the filter 26. The next modulatingprocess carried out in modulator 27 served to reconvert these side bandsinto frequencies that may be recorded. The carrier for modulator 27,however, is so chosen that the lowest frequency in the side bands, inthis case 2000 cycles, is well removed from the lower range which is sodifficult to record. The record made in accordance with this system willtherefore be a representation of all frequencies between 0 and 6000cycles, but will actually consist of a wave form of inverted frequenciesranging from 2000 to 8000 and upward.

A suitable system for reproducing these records is comparatively simpleas shown by Fig. 4. An electromagnetic pick-up 32 may be employed toimpress upon a simple modulating device 33 currents corresponding to therecorded sounds. The carrier frequency generating system is arranged tosupply a carrier of a frequency equal to the difference between thecarriers used in recording which in this case is 8000 cycles, so thatthe lower side band in the output circuit modulator 33 will comprise thesame frequencies as were impressed uponthe microphone of the recordingsystem, namely 0 to 6000 cycles. The receiver 34 should benon-responsive to the 8000 cycle carrier or to its upper side band andof a type which satisfactorily repro duces the part of the range between0 and 6000 cycles which is of particular interest. For instance, amoving coil type receiver would be preferable for low frequency sounds,but in cases where reproduction of frequencies below the range ofaudibility is desired, the receiver 34 might take the form of a visualindicating device responsive to these low frequency variations Aspreviously pointed out, if a zero frequency variation could be producedto actuate the pick-up device 11 of Fig. 1 it would appear on the recordas carrier frequency of 8000 cycles per second. Similarly in the systemof Fig. 3 zero frequency is recorded asthe difference between thefrequencies of the carriers employed or 8000 cycles. A variation of onecycle per second would therefore appear as 7999 cycles in the record.

When a system, such as that shown in Fig. 4 is used to reproducefrequencies in the neighborhood of or below the limit of audibility, itis evident that any difference in phase between the carrier frequencycomponents on the record and the carrier wave supplied locally will giverise to distortion. A micrometer screw adjustment 35 is thereforeprovided for the laminated core 3 to accurately adjust the phase of thelocal carrier;

What is claimed is:

1. A method of recording and reproducing high quality sound recordswhich consists in recording sound waves as a side band of a modulatedcarrier the frequency of said carrier being so chosen that the resultingside band is displaced on the frequency scale and occupies a range overwhich the response characteristic of the recording and reproducingmechanisms may be made substantially linear, reproducing and modulatingsaid reproduced side band with a carrier wave of a frequency whichvaries in accordance with the speed of the reproducing turntable.

2. A method of recording and reproducing a band of frequencies extendingdown to Zero frequency which consists in translating said band intoother frequencies representative of said band, recording said otherfrequencies, reproducing said other frequencies and re translating saidother frequencies into said band.

3. A method of recording and reproduc ing a band of frequenciesextending down to zero which consists of recording said band as aninverted displaced side band of a carrier wave reproducing said sideband and inverting and replacing said side band within the rangeoccupied by the original frequencies.

4. In a wax recording and reproducing system adapted to reproduce soundwaves from a recorded side band of a modulated carrier, a method ofmaintaining the reproduced tones in their proper relative phasepositions irrespective of changes in turntable speed which consists indemodulating said side band with a carrier wave generated by meansassociated with said turntable.

5. In a sound reproducing system a record of a sideband of a carriermodulated with sound frequencies, a turntable carrying the record, meansassociated with the turntable for generating a current of the frequencyof the unmodulated carrier, means for reproducing the recordedfrequencies, and means for combining said frequencies and the carrierfrequency to reproduce the original sound frequencies.

6. In a sound reproducing system a record of a sideband of a carriermodulated with sound frequencies, a turntable carrying the record,electromagnetic means associated with the turntable for generating ademodulating carrier wave, means for varying the phase of the carrierwave with respect to the sideband, means for reproducing the recordfrequencies, and means for combining said frequencies and the carrierfrequencies to reproduce the original sound frequencies.

7. In a sound recording system, a sound responsive device fortranslating sounds into electric currents, means for modulating saidcurrents, a record blank, a turntable for revolving said record blank,recording means responsive to the output of said modulating means andmeans associated with said turntable for generating a carrier wave forsaid modulating means.

8. A method of recording a band of fre quencies extending down to zerofrequency which consists of modulating a carrier with frequencies todisplace them in the frequency scale, zero frequency being representedby the unmodulated carrier, eliminating the undesired products ofmodulation, modulating a second carrier with the displaced frequenciesto procure a lower sideband in the optimum recording range and recordingsaid lower sideband.

9. In a sound recording system, a sound responsive device fortranslating sounds into electric currents, means for modulating thecurrents, a record blank, a magnetic turntable having a slottedperiphery for revolving the record blank, recording means responsive tothe output of the modulating means, and means for generating a carrierwave for the modulating means comprising a coil having a magnetic corein close spaced relation to the periphery of the turntable, a harmonicgenerator for multiplying the frequency generated in the coil, and meansfor selecting the desired multiple frequency.

10. In a recording system, a source of carrier frequency, a balancedmodulator, means for recording a portion of the output of the modulator,and means responsive to a constant acoustic pressure in one directionfor unbalancing the modulator and permitting the carrier frequency toactuate the record ing means.

In witness whereof, I hereunto subscribe my name this 2 day of May,1928.

JOSEPH W. HORTON.

